For use as recording media in such disk recording-playback devices, magneto-optical disks have been developed which permit rewriting and have a great memory capacity and high reliability. Such disks have found wide use as external memories in computers and audio visual devices. Developed especially in recent years are techniques for achieving improved recording densities by forming lands 17 and grooves 18 alternately on the signal bearing surface of a magneto-optical disk 1 as shown in FIG. 20 and recording signals on both the land 17 and the groove 18.
The lands 17 and the grooves 18 are wobbled as illustrated, and the wobbling frequency is a predetermined center frequency as frequency-modulated. A wobble signal is detected by signal reproduction, and the rotation of the magneto-optical disk is so adjusted that the wobble signal has the center frequency at all times, whereby constant linear velocity control is realized. Various items of information (wobble information) such as address information are contained in the wobble signal which is frequency-modulated as stated above. Various control operations are realized based on the wobble information at the time of signal reproduction.
With the disk recording-playback device of a laser-pulsed magnetic field modulation type, a laser beam is projected onto the disk for signal reproduction, and a laser beam is also projected onto the disk for signal recording, and the disk is heated locally. Furthermore, with magneto-optical disks using magnetic super resolution, signal reading is started whereupon the temperature in a beam spot region reaches a predetermined value while laser power for signal reproduction is increased. The laser power for signal reproduction is set lower than the laser power for signal recording, so that there is no likelihood that the recorded signals are damaged along with signal reproduction.
With the disk recording-playback device, power (recording power) of the laser beam for signal recording and power (reproduction power) of the laser beam for signal reproduction have optimum values, respectively. If a power differs from the optimum value, a bit error rate of the reproduced signal increases. If the bit error rate exceeds a given prescribed value, difficulty is encountered in performing a normal reproduction operation (see FIG. 14). Accordingly, already proposed is a method for, in the system's initiation into operation, reproducing signals and calculating the error rate while the reproduction power is gradually altered, or recording signals and calculating the error rate of the reproduction signal while the recording power is gradually altered for test tracks pre-provided on the disk, and thereby retrieving an optimum reproduction power and a recording power having the lowest error rates, respectively.
However, with the disk recording-playback device described, the step width in the alteration of the reproduction power and the recording power is required to be set as small as possible in order to retrieve the optimum reproduction power and the optimum recording power with high accuracy, with the result that a long period of time is required for the retrieval, entailing the problem that the device's initiation into operation takes much time.
An object of the present invention is to provide a disk playback device which is capable of jumping to signal reproduction in a short period of time and reproducing signals with high accuracy at all times by setting the optimum reproduction power for each disk.